The invention relates to change speed transmissions in general, and more particularly to improvements in transmissions of the type known as continuously variable transmissions (CVT) which can be utilized with advantage in the power trains of motor vehicles, e.g., to transmit torque from the output element of a prime mover (such as the camshaft or the crankshaft of an internal combustion engine) to the front and/or rear wheels of a motor vehicle.
A continuously variable transmission normally comprises a first shaft which can receive torque from a prime mover, a second shaft which is or can be parallel with the first shaft, an adjustable pulley or sheave on each of the two shafts, and an endless flexible element (such as a chain or a belt and hereinafter referred to as chain) trained over the two pulleys to transmit torque from the first shaft to the second shaft when the first shaft is driven by the prime mover. The two pulleys are adjustable and, to this end, each pulley comprises a first conical flange which is affixed to the respective shaft, and a second conical flange which is rotatable with the first flange and is movable axially relative to the respective shaft toward and away from the respective first flange. Such adjustability of the pulleys enables the chain to move one of its looped portions radially inwardly toward one of the shafts while its other looped portion moves radially outwardly and away from the other shaft, or vice versa.
It is already known to make at least one flange of each adjustable pulley of one or more parts at least one of which consists of a metallic sheet material. This contributes to lower cost and to a reduction of the overall weight of the transmission. The axially movable flanges normally cooperate with cylinder and piston units which define with the movable flanges one or more plenum chambers for the reception of a hydraulic fluid which compels the respective mobile flange to move axially toward the associated axially fixed flange. Such movability of the axialy movable flanges is utilized to establish a desired amount of friction between the chain and the adjacent conical surfaces of the flanges (this ensures that the chain shares the angular movements of the flanges or that the chain is entrained with a desired degree of slip). Axial movability of one flange of each pulley is further utilized to select a desired transmission ratio by moving one looped portion of the chain away from the periphery of the respective shaft while the other looped portion of the chain is compelled to move toward the periphery of the associated shaft.
Continuously variable transmissions are often preferred over automatic transmissions which employ a hydrokinetic torque converter in combination with a socalled bypass or lockup clutch. The reason is that a power train employing a continuously variable transmission affords a greater comfort to the occupant or occupants of the motor vehicle because the shifts into different gear ratios invariably take place gradually without any or without appreciable shocks. Moreover, the utilization of a continuously variable transmission in the power train entails substantial savings in fuel requirements of the motor vehicle.
Published German patent application Ser. No. 43 42 736 A1 discloses a continuously variable transmission wherein the axially shiftable flange of each adjustable pulley is located between the corresponding axially fixed flange and a support which is affixed to the respective shaft. The support and the adjacent axially movable flange define a plenum chamber which can receive pressurized hydraulic fluid to move the axially movable flange toward the respective fixedly mounted flange. The flanges are or can be made of a metallic sheet material, and this can entail considerable savings in the initial cost of the adjustable pulleys. However, the axially movable flanges and the adjacent fixedly mounted supports are designed to define a single plenum chamber for each of the two pulleys. This is unsatisfactory in those types of continuously variable transmissions wherein each axially shiftable flange must be adjusted by a first unit to select the frictional engagement with the adjacent portion of the chain as well as by a second unit which enables the transmission to select the desired speed ratio, i.e., any one of an infinite number of different speed ratios.